This invention relates to a display control device for controlling, in accordance with an application program for a CRT display, for example, such displays as a liquid crystal panel display and a plasma panel display which have a different display timing from that of the CRT display.
With the recent trend of reducing the size of the office automation equipments such as personal computers and word processors, panel type displays such as liquid crystal and the plasma displays are taking the place of CRT displays.
Meanwhile, such panel type displays as mentioned above usually have a different display timing from that of the CRT displays. Therefore, in order to drive the panel displays by an application program designed for the CRT displays, the following methods have been adopted:
The first method is to set, in accordance with an initially set program, contents of a timing control register of an existing CRT controller so that they fit with the panel timing.
The second method is to write data produced from the CRT controller at the CRT timing in a frame buffer and then deliver it out in accordance with the panel timing.
In the first method, it is necessary to change the program of the timing control register in the CRT controller when changing its display mode such as switching of the resolution and the graphic/text in accordance with the application program. However, since the application program is usually designated for the CRT display, if such rewriting of the register program is allowed, the timing for the panel display set by the initial setting will be impaired.
Therefore, it has conventionally been practiced to prohibit rewriting of the timing control register in subsequent processing by the application program by providing a protect flag in order that the register program for the timing control can not be rewritten by the application program. In this case, however, the display mode can not be changed.
It has also been practiced to provide a local CPU or logic circuits additionally in order that it can recognize mode change and after that change the CRT timing to the timing for the panel display and finally write it in the controlling register in the CRT controller. In this case, however, there arises the problem that the cost of the components is increased.
In the second method, there is also the problem that the cost of the whole system is increased as in the first case, as it necessiates the frame buffer and a dot clock reproduction circuit (PLL circuit) for the panel display.
Therefore, it is an object of this invention to provide a display control device capable of smoothly controlling, in accordance with the application program, plural displays that have different timing from each other, without causing a substantial increase in the cost of components.